1. Field of the Invention
The present invention relates to a method for inspecting a photomask employed for replicating a circuit pattern onto a semiconductor wafer in the process for producing a semiconductor integrated circuit, and more particularly to a method for inspecting defects such as the presence of foreign particles affecting the replication of the pattern.
2. Description of the Prior Art
Recent development of finer and denser integrated circuit pattern has stimulated the widespread use of reduction projection exposure apparatus. In such apparatus, in which a pattern on a photomask called reticle is projected in a size reduced to 1/5 or 1/10 of the original size, foreign particles such as dusts on the reticle are replicated in reduced size in all the step-and-repeat exposures onto the wafer as long as they have a certain size and a certain optical density. In order to avoid such inconvenience, the resist image printed on the wafer has conventionally been observed, in trial manner, under microscope by human eyes to detect eventual replication of the foreign particles. This method however requires an extremely tiring work for the eye, for 2 to 3 hours at maximum per reticle, and may eventually overlook the foreign particles present. These drawbacks have only recently been resolved commercially by an apparatus which scans the reticle with a laser spot and detects the scattered light to identify the presence and dimension of the foreign particles, as disclosed in the Japanese Patent Application laid open No. 62543/1983, corresponding to the U.S. Pat. No. 4,468,120. Such apparatus is capable of automatic inspection within a short time, and detecting almost all foreign particles of a size large enough for replication. However the inspecting method employed in said apparatus is designed to satisfactorily distinguish foreign particles from the chromium layer, so that foreign particles lying flat on the chromium layer, if large enough in area, are sometimes mistaken as the chromium layer and not detected.
Also since this method does not directly inspect the replicated pattern, all the foreign particles of a size that may be replicated have to be detected. There has therefore been observed a tendency of detecting even small particles which are in fact not replicated and thus requesting excessive cleanness to the reticle.
Apart from the above-explained inspecting method, there is also considered a totally different inspecting method of overlaying and replicating a reticle to be inspected and another reticle, in which light and dark patterns are inverted, in succession onto a photosensitive material, and inspecting, after development thereof, the presence of spot patterns resulting from lack of exposure. This method has however suffered from insufficient precision in registration between a latent image formed by the first exposure through the reticle to be inspected and a projected image of the inverted pattern of the other reticle to be exposed next time, mainly due to a fluctuation in the positioning of the reticle in the exposure apparatus and to a drift in detecting the position of a stage supporting the wafer. In case the precision of registration of the first and second exposures is insufficient, there may be considered a method of increasing the exposure to increase the width of overlapped exposure of photoresist at the pattern edge, thus exposing the photoresist where the pattern edges are positionally aberrated to apparently cancel such aberration before the unexposed areas are inspected. In this method, however, the foreign particles present on the pattern edge become harder to be replicated and are often or totally overlooked at the inspection.
Besides, even if the positional registration is achieved with an optimum precision, such satisfactory registration cannot be obtained over the entire projection area if two reticles are exposed on different exposure apparatus because of the slightly different image distortion between the apparatus.
Finally, the detection of spot patterns formed on the photoresist layer has to be made by the eyes through a microscope with an objective lens of a high magnification and eventually through a television monitor, so that this method is still associated with a drawback of requiring visual observation in which the required labor is not significantly alleviated since this method only provides a simpler form in the objects of inspection. This method has not been applied to the practical use, since the above-mentioned drawback has become more serious with the recent development of finer circuit patterns.